"We mapped the entire trench, from its northern end at Dutton Ridge, all the way to where it becomes the Yap Trench in the south," explained Dr Jim Gardner from CCOM, which is based at the University of New Hampshire.

"We used a multibeam echosounder mounted on a US Navy hydrographic ship.

This instrument allows you to map a swath of soundings perpendicular to the line of travel of the ship. It's like mowing the grass.

And we were able to map the trench at a 100m resolution," he told BBC News.

The distance to the bottom of Challenger Deep has an error associated with it of about plus or minus 40m.

The figure of 10,994m is slightly less than some other recent measurements in the modern era, but they are all broadly similar.

A location in the trench about 200km to the east of Challenger goes almost as far down.

It is extraordinary to think that both Challenger and HMRG extend deeper below sea level than Mount Everest rises above it.

Dr Gardner said his team's survey put a huge effort into getting the "sound speed profile" of the water column correct - this measure of how the echosounding signals speed and slow as they descend is the largest source of error in the measurement.

The US State Department funded the study because it wants to know whether the exclusive economic zone encompassing the American territories of Guam and the Northern Mariana Islands can be pushed out beyond its current limit of 200 nautical miles (370km).

"As soon as the Pacific Plate starts bending down, it cracks that old, old crust. That crust is really brittle. It cracks right through the seamounts. Certainly in the Mariana Trench, the seamounts get splintered and whittled away, and then get subducted. What I don't see are remnants of seamounts being accreted to the inner wall of the trench."

What is evident, however, is the pile of material this creates across the axis of the trench in a number of places.

Dr Gardner describes four "bridges" that stand as much as 2,500m above the floor of the depression.

'Race to the bottom'

The survey is also highly topical in that four teams are about to send manned submersibles into the trench to explore its depths.

So far, only two humans have visited Challenger Deep - Don Walsh and Jacques Piccard in the research bathyscaphe Trieste in 1960.

But those that are racing to return to the deepest of the deep are Virgin's Oceanic submarine, which will be piloted by Chris Welsh; Triton submarines, which is based in Florida; James Cameron is said to be backing another effort in a bid to film the Mariana Trench; and DOER Marine, which is backed by Google's Eric Schmidt and oceanographer Sylvia Earle.

There is renewed interest in the trench and new submersibles will soon dive its depths.

These missions will be a huge gamble for those involved, both in terms of finance and reputation, as well as posing a serious safety concern for the pilots.

Dr Earle told BBC News: "In 1960, Don Walsh and Jacques Piccard knew only that they were descending in the bathyscaphe Trieste to what was thought to be the deepest place in the sea - the Challenger Deep in the Mariana Trench. "

The terrain was unknown, unmapped - truly a dive into a deep mystery.

Today, as explorers begin to return to that deepest place, they can do so with a high-resolution map in hand, thanks to technologies that did not exist half a century ago.

"In broad strokes, the ocean's great mountain ranges, valleys are now defined in maps derived from satellite observations and sonar swaths from research vessels, but only about 5% of the ocean has been explored and mapped with the detail comparable to Earth's above-ocean terrain, or the surface of the Moon, Mars, or Jupiter. "

In 1960, it seemed that nothing humans could do could alter the nature of the ocean - or if we did, it wouldn't matter.

Now we know that the ocean is Earth's 'life support system', the blue heart of the planet, the key to climate, weather, planetary chemistry.

We also know that the ocean is in trouble owing to what we are putting into it - and what we are taking out.

"High-resolution maps of the iconic Mariana Trench may inspire action to fill in the blanks for the rest of Earth's surface - under the sea. At the very least we should have a good map of the part of the Earth that keeps us alive."